A well-known NAND flash memory device includes a memory cell array, a row decoder, and a page buffer. The memory cell array includes a plurality of word lines extending along rows, a plurality of bit lines extending along columns, and a plurality of cell strings respectively corresponding to the bit lines.
A string select line, the word lines, and the row decoder connected to a common source line are disposed on one side of the memory cell array, and the page buffer connected to the plurality of bit lines is disposed on the other side of the memory cell array.
As multi-level cells (MLCs) having a cell voltage distribution width of a multi-level are developed, a method of reducing the width of cell threshold voltage distributions may become a problem.
A conventional program method employs an Incremental Step Pulse Programming (ISPP) method of reprogramming a cell, which does not exceed a verify voltage level at a specific voltage level, by raising the voltage of the cell as high as a specific step and may present additional problems.
FIG. 1A illustrates a conventional ISPP voltage supply level. As shown in FIG. 1A, a program may perform at an initial program start voltage V1, and a program verification is then performed at a verify voltage PV. The program may perform again on cells that have not been verified by increasing the program voltage as high as a predetermined voltage step Vs or by raising voltage as high as the voltage step Vs. If the cells are not verified even after the program is performed up to a maximum program voltage Vn in this program process, program fails.
Meanwhile, a memory device having a multi-level cell may be configured to store plural pieces of bit data in one memory cell. As the number of bits for storage increases, the width of cell threshold voltage distributions reduces.
In a program process of a NAND flash memory device, the threshold voltage of a cell into which data is already programmed is read to have a higher value due to capacitance between cells in a process where adjacent cells are programmed. This is called an interference phenomenon.
A degree in which cell threshold voltage distributions are changed by the interference phenomenon is great as a change of the threshold voltage of peripheral cells or adjacent cells is increased. In order to reduce the degree in which the threshold voltage of adjacent cells is changed, a soft program on-chip method of programming an erased cell at a low voltage with the threshold voltage of the erased cell being erased in order for the threshold voltage of the cell to have a negative value, but an absolute value thereof to be low, and so on are used.
FIG. 1B illustrates a change of threshold voltage distributions depending on a soft program of erased cells. As shown, threshold voltage distributions of an erased cell 101 may be changed to have a low absolute value 102 while maintaining a negative value through a soft program. The soft program operation may be performed on the entire cells using the same program voltage, wherein a verify voltage is set to 0V. When program pass occurs even in 1 bit, the soft program operation is completed. If the above soft program is performed, an erased cell has threshold voltage distributions of about −1V. The threshold voltage distributions of the erased cell may reduce the threshold voltage change phenomenon due to the interference phenomenon up cell program.
However, although the soft program method is employed, the threshold voltage of a cell may not be prevented from being changed depending on a program operation of adjacent cells with the threshold voltage of the cell is first programmed.